A diversity handover communication technique, wherein a mobile station moves across the boundary between cell sites of different base stations while continuing to communicate with the base stations, is described in Japanese patent application No. 6-106953. This application states a method in which a base station creates reliability information based on the state of radio frames received from a mobile station, and attaches the reliability information to each radio frames. Then, a diversity selection process is carried out in the communication network.
Japanese patent application No. 6-210193 discloses another diversity handover method in which when communication is made between a mobile station and a superior system, frame identification information is used, to prevent the occurrence of skipping or overlapping of frames during diversity selection of the frames due to the difference in delay of frame transmission through different base stations, and thus secure diversity handover is ensured.
However, these methods have following problems.
(1) In the method disclosed in Japanese Patent Application No. 6-210193, when a mobile station (MS) makes a communication through a mobile switching center (MSC), frame identification numbers are used to absorb differences in delay arising during the passage of frames through different base stations, and maximal-ratio combining or diversity selection of resulting frames is achieved. For MS to absorb differences in delay of downlink frames, it is necessary for MS to have a buffer with a considerably large capacity. This makes it difficult to reduce the size of a responsible terminal. Further, as this method requires frame identification information to be exchanged between different radio zones, the communication system it promises to realize will be inefficient because it will be not able to effectively exploit the capacity allowed to radio routes.
(2) In the conventional frame receiving systems, there was no attention paid for the difference in delay of frame transmission according to the type of involved service, and thus sets a fixed maximal transmission delay independent of the type of service currently involved. Accordingly, even when a transmission mode is introduced which allows different transmission delays according to the type of service (for example, Type 5 or Type 2 of ATM), a receiver must respond with a fixed, too long delay to frames of service which does not require such a long delay.
(3) The conventional frame receiving system regards as fixed the maximal transmission delay arising as a result of the passage of frames through nodes and links, and thus it can not meet the situation where an unexpected transmission delay arises owing possibly to changes in transmission state or in traffic. It causes disconnection of the communication in the presence of such delay.
(4) In the conventional handover process, as communication quality is solely determined by the transmission condition through the radio link, it can be monitored by the radio receiver connected to the link. However, in diversity handover, communication quality is obtained as the outcome of maximal-ratio diversion or diversity selection of frames from all branches involved in the handover, and thus it can not be monitored only by a radio receiver.
Maximum ratio combining of frames is a technique whereby MS receives downlink frames from a plurality of BSs, and combines received signals in such a way as to improve communication quality by site diversity effect. This technique is also utilized by a single BS which combines uplink frames from MSs incoming through a plurality of TRXS.
Namely, in handover involving a plurality of sectors in a zone governed by a BS (intracellular, inter-sector diversity handover), combining of uplink radio frames is performed by the BS according to maximal-ratio combining.
On the other hand, diversity selection is applied to combining of uplink radio frames in diversity handover involving a plurality of BSs. Uplink radio frames coming by way of a plurality of BSs are given reliability data different according to the routes they pass, and a diversity handover trunk chooses a frame having the best reliability information.
The reason why maximal-ratio combining is not applied for the combining of uplink radio frames in handover involving a plurality of BSs is to prevent transmission of a vast amount of information required for maximal-ratio combining through routes connecting the plurality of BSs and MSC, and thereby to prevent congestion of traffic. Diversity selection, as compared with maximal-ratio combining, does not require much reliability information for combining, although it allows only a low gain.
(5) With the conventional technique, when an out-of-sync (out-of-synchronization) state arises, BSs, whenever they detect it, inform of it to MSC processor through their own control links independently of each other. In diversity handover system, control is made such that the power required for transmission of uplink frames from MS becomes most efficient for a certain BS. Therefore, the other BSs which are not objects of power control may often inform MSC of out-of-sync states. Thus, a vast amount of control signals are transmitted through routes connecting BSs and MSC processor, and an overload is imposed on the processor.